1. Surface Instability in Soft Materials Rui Huang University of Texas at Austin

2. Outline  Elastomer (rubber) block  Elastomer bilayer (thin film) or graded stiffness  Polymer gels  Electromechanical instability of dielectric elastomer  A simple buckling problem

3. Elastomer block Wrinkling or creasing?  Biot’s linear perturbation analysis for wrinkling  Nonlinear stability analysis for creasing (Hohlfeld and Mahadevan, 2011; Hong et al., 2009)  From wrinkles to creases (Cao and Hutchinson, PRSA 2012)  Effect of surface energy (Chen et al., 2012)

4. From instantaneous to setback creases Diab, Zhang, Zhao, Gao and Kim (2013)

5. Elastic bilayers: from wrinkling to folding Cao and Hutchinson, JAM 2012

6. Effect of pre-stretched substrates Cao and Hutchinson, JAM 2012

7. Experiments Sun et al., 2012 Pocivavsek et al., 2008

8. More bifurcations Brau et al., 2010

9. Gels: Swell-Induced Instability Trujillo et al, 2008. Tanaka et al, 1987  Wrinkles or creases?  Critical condition  Characteristic size  Effect of kinetics Abundant experimental observations, but lacking fundamental understanding.

10. Bilayer gels: two types of instability A B Wu, Bouklas and Huang, IJSS 50, 578-587 (2013).  Type A: soft-on-hard bilayer, critical condition at the short wave limit, forming surface creases;  Type B: hard-on-soft bilayer, critical condition at a finite wavelength, forming surface wrinkles first (and then creases).

11. Gradient and kinetics Guvendiren et al, 2009 & 2010.

12. Other geometries Wu et al, 2013. Dervaux et al, 2011. DuPont et al, 2010.

13. Dielectric elastomer membranes: Electromechanical instability Plante and Dubowsky, IJSS 2006. Huang and Suo, 2012.

14. A simple buckling problem? simply supported, but allow vertical displacement x y  At x = 0, buckling amplitude is zero (no buckling)  At x → infinity, unconstrained buckling (long wavelength mode)  In between, short-wavelength mode appears near the end, and transition of buckling mode occurs.  Postbuckling behavior: how would the buckling mode change with position (x) and the compressive strain?

15. From graphene to curtain: Wrinklons? Vandeparre et al., 2011.